Paul Fitzgerald and his family said they are descended from the sixth duke, Maurice, who was supposed to have died unmarried and childless in an insane asylum in the 1920s. They said another man had been confused with Maurice, who was released, moved to the United States, married and had a family.
The chancellor, Lord Falconer of Thoroton, decided that the title should remain with Maurice Fitzgerald, the great-nephew of the U.S. Fitzgeralds' supposed ancestor, The (London) Telegraph reported.
The dukedom, created by King George III, was the most important title in the Irish peerage. The Irish parliament or Dail meets in what was once Leinster House in Dublin, a measure of the family's importance.
In more recent decades, most of the family's hereditary property has been sold, and the current duke is a successful businessman living in Oxfordshire.